I think there are probably a lot of Arduino users out there who either (a) don’t know about interrupts, or (b) don’t know you can use them in the Arduino environment. I will almost certainly be using the Timer Interrupt capability of the ‘duino in an upcoming project that uses Charliplexing (get excited!). I found the following to be an excellent overview and reference in that respect: http://www.uchobby.com/index.php/2007/11/24/arduino-interrupts/

For more “advanced” Arduino users, it helps to keep in mind that at the end of the day, all your source code gets compiled using avr-gcc, so in principle you can do anything with an Arduino that you could do by targetting an ATMega328 (or ATMega168) using vanilla avr-gcc (i.e. use any of the chip’s hardware). For some things it’s harder than for others because the core Arduino libraries implement some of the interrupt handlers (ISRs), but for some things, like Timer2, it’s open season! Other than that you just have to make sure you are not trampling over some other (Arduino) function of the hardware resource you want to hijack :). Have fun!

This is very clever, using an Arduino UNO to target an ATTiny45 or ATTin85 with a sketch! Watch out for assumptions in Arduino libraries, most notably that the assumption of 16MHz operation. And of course there are clearly limitations in terms of available pins on the smaller device. For what it’s worth, Wicked Device Day Counter kits come with an ATTiny85V and a programming header if you want something a bit more sophisticated than a breadboard to play with :)!